TL;DR

Improving the theoretical model and practical design of a Blackburn pendulum.

I would like to make my model and experiment more realistic. Could you help me with the following questions?

1. How do I formally incorporate linear damping using Rayleigh's dissipation function into the Euler-Lagrange equations?

2. Besides Rayleigh damping, are there any other energy losses or real-world effects (such as air drug...) that are not negligible in a university experiment? And what other negligible influences are there on it?

3. Do you have any practical design suggestions or ideas on how to build an apparatus that allows for easy and precise adjustment of the pendulum's effective lengths?

Thank you for all the answers and suggestions